90 Mr. Mater on the Dynamics of Earthquakes. 
out fracture, the earthquake may commence by very gentle shocks, gradually 
increasing in violence, and again diminishing in force.* 
Where the elevated crust consists of horizontally, or nearly horizontally strati- 
fied rock, or of several parallel superimposed masses of independent rock, of what- 
ever sort, those that lie deepest will be first and most bent, and, ceteris paribus, 
will be the first to give way by fracture ; those above them will break in succession, 
and when the area and amount of elevation are very great, each layer or plate of 
the whole elevated crust may be fractured successively at several places, so that 
from a single locus of elevation a number of earth waves may be produced and 
propagated in succession, each constituting a true earthquake shock : nay, even 
after numerous fractures have taken place, the further upheaval and tilting over 
of vast masses of the now broken up plate will, where its thickness is considera- 
ble, produce renewed mutual pressures and violent constraint, in the directions 
of the diagonals of the several tabular blocks, which will afterwards give rise to 
minor shocks, as in the further progress of elevation, the several masses of the 
ruptured crust are raised and freed from each other, and the compression and 
constraint of their elastic particles thus successively removed. (Plate II. fig. 8, 
a. B. y. ., and Description of Plates). 
Where successive fractures at different, but great, depths, take place in this 
way, two distinct systems of elastic waves, one of them having a vertical, and 
the other an horizontal or largely inclined direction of transit, will traverse the 
mass of the earth’s crust, and be felt upon its surface at once; but the 
amplitude of the waves of the former system will be very small, as compared with 
the waves of the latter, and hence an undulating and oscillatory motion will be 
experienced at the surface, accompanied by a sharp upward jar or vibration at 
the same time,—circumstances which have been occasionally recorded as having 
been observed during earthquakes. 
Such, then, I conceive to be the true origin and nature of the earthquake 
shock. It is produced by any force which disturbs the equilibrium of elasticity 
of the materials constituting the crust of the globe, and it is propagated from the 
locus of its origination, in accordance with the laws of transit of elastic waves 
through such materials. 
* See Alison’s Account of the Chilian Earthquake, in February, 1835, Geol. Trans.; and 
Caldcleugh’s Account, Phil. Trans. for 1835. 
